It is true. Deforestation diminishes fresh air supply, as trees are important oxygen productors. Besides, deforestation coupled with overpopulation, and industrial activities, do synergy (that is, the factors cooperate with each other) to diminish air quality. Other important factors are climatic ones, such as differences in atmospheric pression, which causes strong winds that might increase dust/particles in the air. Although, the most important contemporary factors diminishing air quality are anthropocentric.